Improve and fix Airzone config flow

[Noltari]

Proposed change

Improve and fix Airzone config flow:

• A user can add multiple config entries for the same Airzone device for those Airzone devices that support all systems listing (systemID=0) since _async_abort_entries_match is being checked with CONF_ID always set, but if the Airzone device supports systemID=0, CONF_ID won't be set. This bug was introduced in Support specifying Airzone System ID #69751 c76b21e.
• Extend SYSTEM_ID_SCHEMA from CONFIG_SCHEMA.
• invalid_system_id error refers to CONF_ID.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

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• This PR fixes or closes issue: fixes #
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[gjohansson-ST]

Any particular reason not to display the system id already first time?

[Noltari]

Any particular reason not to display the system id already first time?

Yes, newer devices support setting the system id to 0, which automatically lists all connected systems. Older devices do not support this and the system id has to be specified.
I’m not sure if those older systems support more than one system connected or not.
However, @frenck suggested doing it this way on the referenced PR: #69751 (comment)
And I also think it’s better to do it that way, since most users won’t know what to enter in the config or ir 0 is even a possibility since system id should be 1-32.

[Noltari]

Thanks @bdraco! 🎉